1. Field of the Disclosure
The present invention relates generally to energy storage circuits, and more specifically, the invention relates to circuits that use capacitors to store energy in power supplies connected to single phase AC power sources.
2. Background
Known AC-to-DC power supplies typically have three principal groups of circuits that perform distinct functions: input rectification, bulk energy storage, and DC-to-DC conversion. The circuits that perform input rectification change bidirectional current from a single phase AC power line into rectified input current that flows in only one direction. The bulk energy storage function is typically performed by one or more capacitors that form what is often referred to as bulk capacitance. The bulk capacitance receives the rectified input current, using it to store energy in the form of a voltage on one or more capacitors. The voltage on the bulk capacitance is usually too high in magnitude and not steady enough to power the delicate circuits in electronic equipment. Therefore, a DC-to-DC conversion function changes the rectified voltage from the hulk capacitance into a more suitable form at the output of the power supply. The DC-to-DC converter is usually one of many well-known switching power converter circuits that can produce the desired output with high efficiency.
The rectified voltage on a bulk capacitor typically varies between a peak value and a valley value at a frequency that depends on the type of rectification and the frequency of the AC power line. The peak is typically the voltage at the peak of a cycle of AC input voltage. The valley is determined by the amount of capacitance and by the amount of power demanded by the DC-to-DC converter. The difference between the peak and the valley is the ripple voltage. It is important that the ripple voltage does not get too large at the minimum AC input voltage specified for the power supply because the DC-to-DC converter needs a minimum rectified voltage at its input to produce the desired output.
An additional concern is the value of the valley voltage when the voltage from the AC power line is removed. The power supply is typically required to provide its normal output for a short time after the AC input voltage is removed so that the electronic circuits that receive power from the power supply can perform necessary tasks before the electronic circuits lose power. The amount of time the power supply continues to operate after the AC input is removed is referred to as holdup time. During the holdup time, the DC-to-DC converter must get all its energy from the bulk capacitance. The energy available from the bulk capacitance is proportional to the value of the capacitance and to the square of the voltage on the capacitance.
The capacitors that make up the bulk capacitance are selected to meet several requirements that are influenced by the intended use of the power supply. The physical size of the power supply is strongly influenced by the value and the voltage rating (the maximum voltage that an individual capacitor can reliably withstand) of the capacitors that are part of the bulk capacitance. The cost of the bulk capacitance is also a significant part of the cost of the power supply. For a given value of capacitance, a higher voltage rating translates to higher cost.
The voltage rating is selected for reliable operation at the maximum AC input voltage of the power supply, whereas the capacitance of individual capacitors is selected based on the minimum specified AC input voltage of the power supply. The total bulk capacitance value must be selected to ensure the minimum input voltage required for the DC-to-DC converter is not reached when the power supply is operating from the minimum specified AC input voltage.
Known AC-to-DC power supplies designed to operate from a wide range of AC input voltage (typically between 85 volts AC and 265 volts AC) therefore require physically large capacitors due to the high capacitance values demanded by the minimum AC input voltage combined with very high voltage ratings demanded by the maximum AC input voltage.